In recent years, the environmental problems such as earth warming due to release of carbon dioxide gas accompanying the use of fossil fuels, radioactive contamination caused by accidents in atomic power plants, and nuclear waste have become serious, and the spotlight centers on terrestrial environment and energy. In this situation, a solar cell utilizing sunlight as an inexhaustible and clean energy source, geothermal power generation utilizing a geothermal source, wind power generation utilizing wind power, and the like are in practical use in the world.
The direct current electric power generated using the above natural energy is converted to alternating current electric power by an electric power converting device called an inverter, and supplied to, e.g., a commercial electric power system.
In this electric power converting device, improvement in conversion efficiency is important and power loss in a gate driving circuit is not negligible. To reduce the power loss, use of snubber energy in gate driving is known (See, e.g., Japanese Published Unexamined Patent Application No. 5-344708). Further, regeneration of gate electric power of a converter of a transistor on the power source side of a gate driving circuit is known (See, e.g., Japanese Published Examined Patent Application No. 3-36332). These techniques are generally regarded as electric power regeneration techniques, and unless these techniques are utilized, the gate electric power becomes electric power loss.
On the other hand, as a method of effectively using electric power generated by a solar cell, a single cell converter system is proposed (See, e.g., U.S. Pat. No. 5,660,643). According to this method, power generated by solar cells, which are not serially connected, as a low voltage of about 1 V, is inputted into an electric power converting device, voltage-boosted there and utilized.
In this converter system, the gate driving power and the reduction of power loss are also important. However, any method for reducing electric power loss, appropriate to a gate driving circuit used in a low-voltage area to handle a comparatively small amount of electric power, has not been proposed so far.